This invention relates to a metal article such as an engine piston and method of making same and inserts incorporated therein, and more particularly relates to an article such as an engine piston of which at least a part is a metal matrix composite reinforced by inorganic fibers.
In an internal combustion engine, and more particularly diesel engines, the pistons are generally subjected to very severe operational conditions, and as a result, the provision of a strong constitution, therefore, is very important. In particular, three design characteristics that are very important for an engine piston are: (1) resistance to wear; (2) resistance to burning or seizure, and (3) resistance to thermal fatigue (thermally induced cracking). The areas of the piston which are subjected to the most extreme conditions (high temperature and high pressure), and therefore need maximized design material characteristics, are the upper and lower wall surfaces of the top ring-receiving groove and the top combustion bowl in the crown of the piston.
Accordingly, in the prior art it has been widely practiced, in the case of diesel engines, for the part of the piston from which the top piston ring groove is formed to include a reinforcing insert formed of special cast iron such as NiResist cast iron which has more wear resistant and thermal fatigue properties than the matrix material from which the piston is mainly constructed (e.g., aluminum or aluminum alloy). However, such pistons cast with wear-resistant ring carriers, such as NiResist cast iron, suffer from certain problems such as lower thermal conductivity, poor adhesion of the insert to the piston body and expense of producing and incorporating the insert in the piston. With regard to the combustion bowl area, attempts have been made to reduce the propensity for thermal cracking by modifying the piston matrix alloy and changing the geometry of the combustion bowl itself, but these changes have proven only marginally successful. In some cases, the changes in geometry of the bowl have even reduced efficiency of combustion in the engine.
One further solution has been to provide reinforcing ceramic fibers in the metal matrix of the piston often with the further addition of a surface coating layer of a ceramic. These fibers and surface layers are added to the area of the piston where high temperature and/or anti-wear characteristics are needed, such as in the combustion bowl region and ring area of the piston. The incorporation of the reinforcing fibers in the piston, however, has been difficult to achieve in standard casting practice without the employment of difficult and costly alloying additions, which may lead to reliability problems in operation. Additionally, when the fiber volume fractions needed to gain desired properties of the composite material are high, then severe thermal cracking can occur, especially if solution heat treating of the piston is tried to maximize strength.
Accordingly, it is a principal object of the instant invention to provide a fiber reinforced metal article and method of making same, which is produced with a preformed ceramic fiber insert that will provide good wear resistance, increased elevated temperature strength and desired heat transfer to the article without an additional surface layer, thus being reliable in operation and economical to produce.